This program project renewal application is submitted by investigators who propose to expand and continue their studies on the unique properties of human milk. The five component subprojects and four subcontracts address the biologic consequence of specific milk components or properties through in vivo protocols conducted in animal models and humans. The subprojects generally seek to define and characterize factors in human milk that protect newborn infants from disease. Certain consequences of the luminal milk-gastrointestinal tract interaction that pertain to protection of the infant will be examined. Animals will be utilized when study of human subjects is not possible, but human infants will be studied whenever appropriate. The respective subprojects and subcontracts will consider: 1) the role of soluble milk factors in the prevention of shigellosis, 2) antibody secretion, cytotoxicity and immune regulation, 3) the relationship of the human milk phagocyte to infant health, 4) the role of the secretory immune system in rotavirus infection, 5) isolation and characterization of the protective factor(s) in human milk against heat stable enterotoxin of E. coli, and 6) the role of human milk in the prevention of campylobacter infection.